The efficient operation of boilers and other steam generating equipment requires chemical treatment of feedwaters to control corrosion. Oftentimes this corrosion is a result of oxygen attack of the metal components in contact with the water in the steam generating equipment.
Oxygen attack is unavoidably accelerated by the high temperatures often found in steam generating systems.
The corrosion is in the form of pitting which is a highly concentrated corrosion affecting only a small area relative to total surfaces of the system. This, however, can be a serious problem. causing metal failure even though the metal loss is small and the overall corrosion rate is low.
The severity of oxygen attack is dependent on the concentration of dissolved oxygen in the water, the water pH and the temperature of the system. As water temperatures increase, enough driving force is added to the corrosion reaction that even small amounts of oxygen dissolved in the water can cause serious problems. Oxygen pitting is a serious problem in steam generating systems, even where only trace amounts of oxygen are present.
Deaeration is a widely used method for removing oxygen from an aqueous medium. The deaeration of boiler feedwater can be either mechanical or chemical.
While vacuum deaeration has proven to be a useful mechanical deaeration method for treating water distributing systems, boiler feedwater is treated using pressure deaeration with steam as the purge gas. According to the pressure deaeration method for preparing boiler feedwater, the water is sprayed into a steam atmosphere and is heated to a temperature at which the solubility of oxygen in the water is low. About 90 to 95 percent of the oxygen in the feedwater is released to the steam and is purged from the system by venting.
Mechanical deaeration is considered an important first step in removing dissolved oxygen from boiler feedwater. However, as already noted, as water temperature increases, even trace amounts of dissolved oxygen can cause serious problems. Accordingly, supplemental chemical deaeration is often required.
Hydroquinone is often used as an oxygen scavenger for the chemical deaeration of feedwater. However residual levels of hydroquinone will decompose and cause further acid-induced corrosion.
Therefore, it is often necessary to test boiler waters for residual levels of hydroquinone so that appropriate chemical treatment can be applied to neutralize the corrosive effects of hydroquinone decomposition.